In the production of oil and gas from oil and gas wells salt water ordinarily is also produced. The water often contains high levels of scale-forming ions which tend to precipitate and form scale, particularly at points where chemical compositions and physical conditions abruptly change, as in going from the formation into the wellbore. The result can be the formation of large quantities of scale and precipitated salts which plug the formation near the well bore and restrict the flow of oil and gas therefrom. Scale also forms upon well casing and tubing, surface storage and processing equipment such as pipes, valves, heating coils and tubes, separators, etc. associated with handling of the liquids. When scale is formed, the production rate decreases and ultimately, the whole operation ceases. Reduced heat transfer capability is also another bad effect.
In many oil fields, particularly in the North Sea, the most troublesome scales are barium and strontium sulfates. Once a deposit of these is formed, their removal is difficult and costly even to regain near the original production rate. One approach to solving this problem is to shut in the production well and to soak the well with chemicals, such as the sodium salt of diethylenetriaminepentaacetic acid which dissolves or chelates the barium ion for removal. The treating process requires about one week to perform and must be repeated frequently. The treatment is not only costly but also results in lost oil production and corresponding revenue.
A technique for the inhibition of scale formation is disclosed in Meyers, J. Pet. Tech., June 1985, 1019-1034. Meyers discloses the use of scale inhibitors which are squeezed into the formation where they are adsorbed onto the reservoir rock after which the inhibitor is desorbed from the rock into the produced brine as the treated well is produced and refers to retention of the inhibitor by precipitation with divalent cations such as iron and calcium and notes that precipitation in this way may increase inhibitor retention and reduce the permeability in the region of the well bore. Meyers mentions phosphonates and polyacrylates as scale inhibitors (page 120) but does n however, refer to the possibility of using cross-linked scale inhibiting compositions.
Miles U.S. Pat. No. 3,704,750 discloses a method for introducing a scale inhibitor in to oil well brines by injecting a brine insoluble polyvalent metal salt of the inhibitor into the formation adjacent to the wellbore. The inhibitors which may be used include polyacrylic acid having a molecular weight range of about 17,000 to 50,000 and the metals which may be used to form the insoluble salts include trivalent chromium. An essentially similar disclosure is found in U.S. Pat. No. 3,827,977 the '977 reference. In each case, Miles relies on the use of insoluble metal salts e.g. chromium salts. It has been found that insoluble materials similar to Miles' metal salts which include large amounts of chromium are not adsorbed so readily onto the formation with the result that formation permeability in the region of the wellbore is reduced (as noted by Meyers) and time release characteristics i.e. release of the inhibitor over an extended period of time, are lost.
Emmons U.S. Pat. No. 4,710,303 discloses a method of preventing the formation of barium sulfate scale by the use of low molecular weight polyvinylsulfonates.
U.S. Pat. No. 4,787,455 (Snavely) recognizes the desirability of slow release of the corrosion inhibitor and discloses the use of polyacrylic acids as scale and corrosion inhibitors without, however, recognizing the desirability of using trivalent chromium as a cross-linking agent for the complex.
Kaufman, U.S. Pat. No. 4,518,511 discloses the use of oil based antiprecipitant formulations and discloses the use of emulsified polymeric antiprecipitating compositions based on low molecular weight polymers including acrylates for use in oil well treatment but this patent does not deal with water soluble materials for providing time release characteristics.
A primary object of the present invention is to provide an improved method for preventing the deposit of scale, particularly magnesium, barium and calcium, in oil and gas wells. The present method employs water-soluble complexes of chromium and polyacrylic acids which are capable of undergoing a progressive release of the effective scale inhibitor over extended periods of time. The present method uses a relatively low range of molecular weights to ensure that the inhibitor is water-soluble and, in addition, uses a relatively smaller amount of chromium so as to form a complex rather than a salt of the acid as in the Meyers process.